Sources,preservation,and influencing factors of organic carbon from the Dagu River Basin
In this study,surface water,suspended particulate matters and sediments were collected in the Dagu River of Shandong Peninsula during the dry(May)and wet(September)seasons in 2022 for the determination of total organic carbon(TOC)and nitrogen(TN),stable carbon isotopes(δ13C),biomarkers,and mineralogical parameters.The results showed that the sources of organic matter in the surface suspended particulate matter in the Dagu River in the wet and dry seasons were quite different.The surface suspended particulate matter in the wet season had a negative δ13C(-29.1‰)and a low POC/PN(6.3),and its organic matter mainly came from river phytoplankton;the surface suspended particulate matter in the dry season had a more positive δ13C(-26.8‰)and a higher POC/PN(8.8),and its organic matter were derived from river phytoplankton as well as from C3and C4plants brought in by riverine erosion of the soil,and the source of organic matter in surface sediments were more complicated,in addition to river phytoplankton,the contribution of C3 plants,C4 plants,marine phytoplankton,and sewage source were also mixed.The relative contribution of the terrestrial organic matter in river sediments based on the two-end member model of δ13C decreases from 58%to 0%from upstream to downstream.Principal component analysis showed that the presence of dissolved inorganic nitrogen(DIN)led to phytoplankton blooms in the river corresponding to high levels of POC,chlorophyll-a and short-chain alkanes.The sediments in the estuarine section could absorb more organic matters due to fine grain size and large specific surface area,thus resulting higher TOC content than that in the riverine section.Comparison of the TOC loading in riverine and estuarine sediments showed that the organic matters underwent resuspension during transportation,and the longer oxygen exposure time resulted in the microbial decomposition of about 85%of the terrestrial organic matter into CO2.The results of this study provide data support and references for understanding the source and conservation characteristics of organic matters in small and medium-sized rivers.
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